docs: def pksign(self, conn):
          """Sign a message digest using a private EC key."""
          log.debug('signing %r digest (algo #%s)', self.digest, self.algo)
          identity = self.get_identity(keygrip=self.keygrip)
          r, s = self.client.sign(identity=identity,
                                  digest=binascii.unhexlify(self.digest))
          result = sig_encode(r, s)
          log.debug('result: %r', result)
          keyring.sendline(conn, b'D ' + result)